Posts Tagged ‘Robert Warren’

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What Can’t You Design In SolidWorks? #2

Tuesday, April 2nd, 2013

RC Hovercraft #2 – SolidWorks

To review, I had 4 main design criteria for the Remote Control Hover Craft.

  • Utilize the SolidWorks and SolidWorks Simulation Suite of software to develop and optimize the hovercraft design.
  • The RC Hovercraft’s main components will be 3D Printed using the Stratasys UPrint.
  • Easy to Assemble. I want to make the assembly as easy and as straight forward as possible with concise instructions.
  • For purchases components, use low cost, off the shelf components including the electric motors, electronic speed control (ESC), batteries, and propellers.

I proceeded forward with the design of the Hovercraft using SolidWorks 3D mechanical design software.  SolidWorks allowed me to quickly develop and execute a first pass design, utilizing Multi-Body Parts, In context Assembly Modeling, Sketch Pictures, Fastening Features, Interference Detection, and several other standard options.  All of this came together in an initial design that meets the above criteria.

The design started with the Top Plate part that houses the downward facing fan assembly and gives the craft its overall dimensional size.  I kept the craft under the 8″ by 8″ tray size of the Stratasys UPrint 3D Printer.  The part consists of multi-bodies; one for the plate and the other for the fan housing.  These bodies have minimal tolerance so they are a snug fit when pressed together for final assembly.  This design criteria is so that if the propeller needs to be serviced later total dis-assembly of the craft does not have to take place. Simply pull the fan unit upward out of the top plate.

Top Plate

Top Plate

Exploded View Front

Exploded View Front

Exploded View Back

Exploded View Back

 

The chassis continues with a bottom plate and separating ribs.  The chassis is hollow as the air needs to fill this cavity before exiting out of the skirt.  The skirt is intended to be a bicycle inner tube cut to size with holes cut around the inner bottom portion allowing the air to escape.  The skirt will be held on by two fastening plates and standard hobby store machine screws.

Section View

Section View

The back cowling snaps into place with a Snap Hook.  The Fastening Feature command was used to create this geometry.  The Snap Hook will allow for ease of assembly, and the cowl contains a cross bar with built in motor mount sized for a 9V-11V brushed can motor. The Cowling and Top Plate will make up the mounting location for the dual rudder system.  The system is driven by  an S75 Nano servo available at most local hobby shops.

Cowl

Cowl

 

 

The canopy will cover all of the electronics including the Receiver, two Electronic Speed Controls (ESC), And two Li-Poly 300MAH 11.1V Batteries.  One ESC and battery per motor.  I originally set out utilizing the Sketch Picture and Surfacing to create the canopy structure. This worked out well, however at this time I did not have the electronics in the full assembly.  When trying to accommodate the electronics under the first variation of the canopy I visibly had interference. Luckily utilizing in-context editing and having a well planned design intent, the changes to the canopy allowed for an easy and quick change.

Sketch Picture

Sketch Picture

Interference Original Canopy

Interference Original Canopy

Receiver & ESCs

Receiver & ESCs

 

Batteries, Receiver, and ESCs

Batteries, Receiver, and ESCs

Canopy Design Change

Canopy Design Change

 

 

There is still much to do with the modeling aspect, but for now I have a good working start to the project and a starting point to investigate the flow and stress characteristics of the design.  The next step is to utilize Flow Simulation to verify the lift ability of the motor and propeller combination  for the lift fan and the rear facing fan assembly.

Top

Top

Front

Front

Back

Back

Side

Side

 

 

 

 

 

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Posted in COSMOSWorks, General, Simulation, SolidWorks, SolidWorks 3D CAD, Stratasys, Stratasys 3D Printers, Stratasys Dimension Printing, Tips & Tricks, Uncategorized | No Comments »

What Can’t You Design In SolidWorks?

Monday, February 25th, 2013

RC Hovercraft #1

For this blog series I wanted to design something from scratch.  Not necessarily a new idea but something fun and cool.  My intention is to design a Remote Control Hovercraft from the ground up.

I want to give you a brief description and history of a Hovercraft:

A hovercraft or air-cushion vehicle is a vehicle capable of travelling over variable surfaces, such as land and water.  The hovercraft operates by forcing a high pressure of air between the bottom of the craft and the surface below.  This high pressure of air lifts the vehicle upward essentially “hovering” above the ground on a cushion of air. The first practical design for hovercraft derived from several coinciding inventions in the 1950s to 1960s. They are now used throughout the world as specialized vehicles for transport and other applications.

500px-Hovercraft_-_scheme.svg

  1. Propulsion Propellers
  2.  Air
  3. Lifting Fan
  4. Flexible skirt

YouTube Preview Image

I have specific goals in mind that I want to meet in the design and build of this project.

 

Goals of the Hovercraft Design:

  • Utilize the SolidWorks and SolidWorks Simulation Suite of software to develop and optimize the hover craft design.
  • The RC Hovercraft’s main components will be 3D Printed using the Stratasys UPrint.
  • Easy to Assemble. I want to make the assembly as easy and as straight forward as possible with concise instructions.
  • For purchased components, use low cost, off the shelf components including the electric motors, electronic speed control (ESC), batteries, and propellers.

I am starting from just an idea, and a sketch. We will see where the design leads.

Hover Craft2

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Posted in Case Studies, COSMOSWorks, General, Simulation, SolidWorks, SolidWorks 3D CAD, SolidWorks 3DVIA Composer, Stratasys, Stratasys 3D Printers, Stratasys Dimension Printing, Tips & Tricks, Uncategorized | 1 Comment »

When To Use A Bearing Load

Friday, January 25th, 2013

The use of a bearing load is brought up frequently during training and technical support discussions. I want to elaborate on this topic with a simple example, illustrating when to use and when it is not necessary to use the bearing load.
Let’s step back a minute and talk about what a bearing load is. According to the SolidWorks Help file:

 

Bearing Loads

Bearing loads develop between contacting cylindrical faces or edges of shells.
In most cases, the contacting faces or edges have the same radius. The bearing forces generate a non-uniform pressure at the interface of contact. You can select between a sinusoidal variation and a parabolic variation in the appropriate half-space, as shown in the figure.

Bearing Load Distribution

In contrast, a uniform load does not vary in strength closer to the tangency of the tube. The load is constant across the applied face.
Setup:

We will examine two models and four scenarios in this exercise. The models will be both a solid and a hollow shaft. The shaft dimensions will be 14″ long with a 2″ diameter. Additionally, the hollow shaft will have a wall thickness of 0.125″.
Both the Solid and Hollow tubes were loaded with a distributed and bearing load in order to compare and contrast the results. Both models were held fixed at either end. The load was applied to the entire length of the top half of the shaft in a vertical direction.

Loading
The Results:

Between the distributed and bearing load on the Solid model, there is no difference in stress and displacement. On the hollow tube, the bearing load shows a drastic difference in the displacement compared to the uniform load. The bearing load shows the majority of the load is being focused on the center of the tube.
Model Load Stress Displacement

Model Load Stress Displacement
Solid 2000lb Distributed 3943.6 psi 0.001066 in.
Solid 2000lb Bearing 3943.4 psi 0.001065 in
Hollow 2000lb Distributed 11,534.6 psi 0.003009 in.
Hollow 2000lb Bearing 12,084.9 psi 0.003467 in.

Significant digits are for illustration only.

The stress is 4.5% higher in the ‘hollow shaft – bearing load’ combination as compared to the ‘hollow shaft – distributed load’ example.
Load Solid Stress
Stress Above Displacement Below Solid Distributed Load
Load Solid Disp

Bearing Load Solid Stress

 

Stress Above Displacement Below Solid Bearing Load

Bearing Load Solid Disp

 

Load Hollow Stress

 

Stress Above Displacement Below Hollow Distributed Load

 

Load Hollow Disp

 

 
Bearing Load Hollow Stress

Stress Above Displacement Below Hollow Bearing Load

Bearing Load Hollow Disp
Summary:

In summary, the bearing load should be utilized when dealing with a hollow or thin walled, cylindrical geometry. Utilizing solid geometry the load differences do not affect the results. The solid geometry distributes the load throughout the solid volume and is inherently stiffer. The hollow tube, missing its internal mass, shows a difference in the displacement of the applied load. The uniform load displaced evenly from tangent edge to tangent edge where as the bearing load concentrated in the center.

A bearing load can be applied to solid and hollow cylindrical geometry however it is only necessary for hollow or thin geometry.

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Flow Simulation and the 75 Dollar Question

Friday, June 22nd, 2012

Is it worth the extra 75 dollars for a long tube header versus a short tube?

Let’s start answering this by examining how an exhaust header works, and why you would want one.  Headers are one of the easiest bolt-on accessories you can use to improve an engine’s performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders.

 

To further understand why the exhaust manifold has an impact on performance let’s review the  combustion cycle of a gasoline engine.

  1. The intake stroke-  Starts with the piston at the top of the cylinder.  As the piston moves downward the intake valve opens allowing the air fuel mixture to enter the cylinder.
  2. The compression stroke-  Moves the piston back up to compress this air fuel mixture, causing the ignition of the air fuel mixture to be more powerful.
  3. The combustion stroke –  When the piston reaches the top of the cylinder, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down.
  4. The exhaust stroke- Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the header.

During the exhaust stroke, back pressure robs the engine of power. The exhaust valves open at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. The more resistance there is to the piston expelling the exhaust gases, the greater the power loss.

Once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a four-cylinder engine, all cylinders utilize the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and the ­muffler. The idea behind an exhaust header is to eliminate the manifold’s back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. Old hot-rodder intuition, gut feel, and experimentation lead to each pipe being the same length, and using a two into one set up. Two into one specifies that the pipe leading from two cylinders merge into one.  In the case of a four cylinder, pipes from cylinders 1 and 2 lead to one pipe, and pipes from cylinders 3 and 4 lead to one pipe.  Those two pipes then merge again into the collector. The two into one method “smoothes” the flow through the pipe causing less turbulence when the flow fields merge.  These pipes come together in a larger pipe called the collector. By making them the same length, it guarantees that each cylinder’s exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector. Basically Header=Power, and we all want more power.

The 75 dollar question arose from my sister.  She is considering replacing her stock exhaust manifold with an after-market header, and was wondering what was the best “bang for the buck”.  After researching the topic extensively we found that across all the after-market brands the designs seemed to be the same regarding pipe routing, materials, etc.  So the main question came down to should she buy the “short tube” or “long tube” header?

Both the “long tube” and “short tube” headers have equal length pipes from the engine block to the collector.  Both ran a two into one method.  The long tube header however claims that since it is longer by design there would be less back pressure due to a smoother flow.  The differentiator was about 75 dollars, and the fact that the “long tube” header would need the catalytic converter to be moved and remounted by a muffler shop.  The “short tube” header is a direct bolt in.

I couldn’t resist turning to Flow Simulation to solve this question.

We purchased the long and short tube headers, and removed the stock manifold to be able to accurately take measurements from them.  The models are close but not exact without a reverse engineering tool such as a scanner or arm.

After the models were completed the next step became the boundary conditions.  I was able to find a good reference guide located on line from www.donaldsonexhaust.com.  Given the engine Horsepower, cubic inch displacement, and operating RPM I was able to determine Intake airflow, and exhaust gas flow in CFM.

This calculated the exhaust gas CFM to be 520.00 CFM, or 130.0 CFM per port. Please see the hand calculations below.

Yes Engineers Still Do Hand Calcs

Knowing the CFM of the exhaust leaving the cylinder allows us to compare pressure drop from the inlet to outlet across the three manifold models.  The stock exhaust will be the base line for comparison.

 

Model Set Up:

 

Inlet Condition:                130 CFM per inlet port

Outlet Condition:             Environmental Pressure

Surface Goals:                   Each Inlet Goal – Static Pressure / Mass Flow Rate

Outlet Goal – Static Pressure / Mass Flow Rate

Results:


Stock Flow Path

Stock Pressure Gradient

Short Tube Pressure Gradient

Short Tube Flow Trajectories

Long Tube Pressure Gradient

 

Summary:

 

The “short tube” header is hands down the best value.  Both after-market headers showed a drastic decrease in pressure drop over the stock manifold however, the “long tube” header only had an edge over the “short tube” pressure by 0.019 PSI.  As a bonus the “short tube is a direct bolt in, not requiring the existing catalytic converter to be moved.  As Engineers we are always worried about time and money, and are often faced with a decision regarding these two factors.  From my engineering background and proof provided by flow I recommended the “short tube” header.

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Posted in Case Studies, COSMOSWorks, General, Simulation, SolidWorks, SolidWorks 3D CAD, SolidWorks Training, Tips & Tricks, Uncategorized | No Comments »

Model Aircraft Control Surface Spacing and SolidWorks Flow Simulation

Saturday, August 27th, 2011

June 2011 AMA (Academy of Model Aeronautics) Model Aviation magazine had an interesting article Titled “Two of the Big Five model misadjustments” written by Dean Pappas. The two misadjustments were Hinge Gap, and Lateral Balance.

The article caught my attention specifically due to the “flow” diagrams drawn in the article explaining airflow over the wing section and aileron control surface. In the below diagram, taken from the article, Dean explains three cases of air flow relative to a control surface.

  1. Clean airflow at the neutral control surface desired for level flight.
  2. When up or down direction is applied to the control surface the air opposite the control surface direction of travel redirects the flow to reattach. This case shows a tight fit between control surface and main structure.
  3. Hinge gaps allow high-pressure air to leak from one side to the other. This weakens airflow on top of control surface partially destroying the bottom airflows ability to rejoin it. The result is poor control surface response during slow speeds.
AMA Article Diagram

AMA Article Diagram

According to the article ”The high pressure on top, as shown would leak through, given a chance. That chance would be a gap in the elevator and control surface. The result is a flat sheet of air that squirts through the gap and distorts the outside of the hinge line. This reduces the effectiveness of the elevator and creates extra drag.”

This section piqued my interest as the hinge gap shown is very large, probably for demonstration purposes. Being an avid RC aircraft modeler I suspected that the small gaps I have in my personal aircraft’s control surfaces may not cause this affect. My hypothesis is that a very large unrealistic gap will cause this affect however using standard hinge techniques this affect will not be as dramatic as the article states. According to the article large hinge gaps can be sealed with strips of MonoKote covering resolving the problem. MonoKote is a heat shrink Mylar covering that is a standard in RC Aircraft construction.

This blogs purpose is to investigate the hinge gap spacing required to cause an airflow disturbance and air leak through the gap area.

Before we get into the model specifics let’s talk a little about aircraft wing terminology. Below is a diagram explaining common wing dimensions and terminology. The chord length is the distance from the leading edge of the wing or elevator to the trailing edge. The model used in this Flow Analysis is a 5.5″ main wing chord. 0.5″ of the chord is the control surface . The airfoil is symmetrical so the upper and lower camber are equal. The model consists of an extruded wing section with one hinge placed in the middle of the wing.

Airfoil.svg

Three hinge types are standard in the RC modeling community. All hinges are typically spaced evenly across the control surface.

  • Standard plastic barrel hinge comprised of two halves held together with a pin. The hinge is typically screwed or glued into place with the barrel tight against both mating sections. Hinges are typically 0.25″-0.5″ wide by 0.5-1″ total length. The barrel typically ranges from 0.0625″-0.125″ in diameter.
  • CA hinges are flat woven wicking material that is inserted into a slot cut in the components. No or little gap is present with this style hinge. CA or Cyanoacrylate glue is used to wick through the hinge and bond with the hinged components.
  • MonoKote hinges are seldom used in modeling except for small aircraft. The MonoKote hinge is typically a strip of MonoKote that is applied to the top and bottom of the hinge area.

SolidWorks Flow simulation was used to investigate the control surface configurations of four models.

  1. A base line neutral control surface position using a no gap CA hinge Type.
  2. Upward deflected control surface using a no gap CA hinge type.
  3. Upward deflected control surface using a Standard plastic hinge with an 0.0625″ barrel diameter.
  4. Upward deflected control surface using a CA Hinge and gap of 0.25″

All configurations have a 10 ft/second flow rate and a 0 degree angle of attack. The flow analysis was an external flow problem. A localized mesh control was used for each run to capture refined accurate results across the model boundary. All other default conditions were used for the flow setup.

Mesh

Note All plots show a Pressure cut plot and Velocity Flow Trajectories.

Results:

  • The base line model showed a symmetrical pressure on either side of the wing at 14.6 psi and a hinge crossing velocity of 16.45 ft/sec. This is expected results for the area section, hinge, and aileron placement.

Neutral Velocity and Pressure

  • Flow Run Two shows a higher pressure on top of the aileron of 14.696 psi and a lower pressure on the bottom of 14.694 psi. The flow velocity across the top of the control surface drops to 5.9 ft/sec while the bottom speeds up to 11.2 ft/sec. As the article states the air “bends” to re attach to the flow at the trailing edge. This results in a turbulence on the bottom of the control surface aiding in the force of the air on the top of the control surface to push the trailing edge down.

Aileron Up CA Hinge

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  • Flow Run Three demonstrates the 0.0625″ barrel hinge gap and the resulting air flow. The run does show airflow across the gap boundary, however probing the area the velocity in this gap is zero. The flow does extend past the wing trailing edge longer than the non-gap position, however the flow does fully rejoin. The same recirculation under the control surface is seen . The pressure on the end of control surface is however higher at 14.699 psi and lower on the bottom at 14.692 psi. The results show negligible flow through the gap and under most circumstances(slow flight) should not cause loss of control due to bleed through.

Aileron Up 00625 Gap

YouTube Preview Image
  • Run Four had the largest gap similar to the gap in the article’s diagram. The flow results show airflow across the gap boundary and a velocity of the airflow in the gap of 3 ft/sec. The flow does extend past the wing trailing edge longer than the non-gap position and does not rejoin. The recirculation does cause a pressure equalization under and over the control surface. Loss of control surface effectiveness would occur in this scenario.

Aileron Up 025 Hinge Gap

YouTube Preview Image

Conclusions: The article is correct to a point. The gap shown in his diagram would cause a control surface loss of effectiveness, however the gap is way too large to be considered realistic. Most experienced modelers know common practice is to get as tight of a fit between control surface and structure be it a wing, elevator, or rudder. A large gap is not only detrimental but is also unsightly and most modelers avoid them for the aesthetic reasons alone. If a modeler sticks to the new CA hinge or follows correct installation practice for a plastic hinge they will be alright in their flying endeavors.

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Posted in Case Studies, COSMOSWorks, General, Simulation, SolidWorks 3D CAD, SolidWorks Training, Tips & Tricks | 1 Comment »

Copy Settings Wizard For Simulation? Almost.

Monday, August 1st, 2011

Have you ever wanted to copy your system settings for Simultaion? Unfortunatly the copy settings wizard just does not cut it when it comes to Simulation settings.

Here is what you need to do.

You will need to have registry permissions and knowledge of registry editor.

• Edit your registry – Select the ‘Start’ menu and then activate the ‘Run’ application. Type “regedit” and click ‘OK’.
• Navigate to- HKEY_CURRENT_USER\Software\srac\cosmos/works
• Copy Key – Right Mouse Button on the “cosmos/works” folder and select Export
• Import Key -Import the settings to a new computer. Copying this key on another machine should copy all the settings.

Simulation Key

If you’d like this option to be included in the SolidWorks copy wizard then please file an enhancement request from the SolidWorks website

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Posted in Simulation, SolidWorks, SolidWorks Training, Tips & Tricks, Uncategorized | 1 Comment »

Dimension Printed Simulation Verified Blow Off Valve Adapter

Wednesday, June 29th, 2011

Adding a new Blitz Blow Off Valve (BOV) to an aftermarket turbo system lead to no clearance between the valve and the hood of the vehicle.  An adapter was needed to drop the BOV from the high pressure pipe outlet to between the twin cooling fans behind the radiator.

 

High Pressure Pipe Assembly

High Pressure Pipe Assembly

 

BOV Adapter

BOV Adapter

The problem statement is as follows:

A custom adapter was developed to accommodate hood clearance.  Before final fabrication out of aluminum a prototype was “printed” using a Dimension Rapid Prototype Printer.  The printed ABS parts are inherently porous and needed to be sealed in order to hold pressure.  The part was dipped quickly  in acetone and then washed thoroughly with soapy water to seal the pores.  The part was then tested to 110 PSI on a test bench before failure.  A second part was then tested on the car.

Because operating pressure is only 10 PSI, a FOS of 10 was provided by the design.

110 PSI Failure

110 PSI Failure

The second consideration is that the BOV is cantilevered off of the high pressure pipe bung .  The BOV weighs approximately 1/8th of a pound. Adding this to the loading still produced a FOS of   5.

Combined Load

Combined Load

Simulation verified the physical test results and showed that the printed part holds up to the design requirements. The printed ABS adapter works so well an aluminum version was never fabricated.  110 passes down the 1/4 mile drag strip, 1000′s of miles, and 4 autocross seasons, and the little plastic adapter keeps on going.

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Design Changes To A Popular Bath Toy Using Simulation Premium

Tuesday, May 31st, 2011

Recently for his 1st birthday my son received an interesting bath toy.  The toy has an electric pump that flows water from the bathtub out the spout through a suspended set of cups.  One of the cups funnels the water to a spinning wheel.  The other disperses the water through small holes, and the last has a floating center that rises as the water collects in the outer cup.

Bath Toy

What I noticed is that the plastic arm that suspends the cups under the water flow may require a design change.  The issue arises due to repeated addition of force in the arm from my son. Although the arm is plenty sufficient to hold the plastic cups as designed it is not equally as designed for a 1 year old to repeatedly pull down on the end of the arm to remove the cups.  Repeated addition of this force has caused plasticity in the arm at the connection to the base.

Let’s examine the geometry to better understand the issue.

The larger ring accepts the cups.  The arm runs between the large ring and the small ring that mounts to the inlet shaft.  A small fillet is used to blend the interface between the arm and the small ring.  In my opinion this fillet is too small.  Here’s why.  Stress = Force / Area.  The smaller the area (fillet size) the higher the stress.

  • The arm was modeled reconstructing the dimensions using caliper and a scale.  Note the dimensions are approximate.
  • The first model has the original fillet size.

Modified Geometry

  • As a design change the second model has a larger fillet added to distribute the stress.

Original Geometry

  • The model material was assumed to be ABS standard with the SolidWorks Library. A more accurate material definition is needed for any real design changes to be recommended.

The problem statement is as follows:

My son takes a bath every night. On average the arm is bent 4-5 times during the course of the bath.  Based on the one month the toy has been in use 30 X 4.5 = 135 applications of force applied so far.  I will analyze the existing and the proposed  geometry for stress based on a specific displacement.  Next perform fatigue analysis on the existing and proposed design for the arm.

The arm is fixed at the recessed ledge where the collar meets the inlet shaft.  A specified downward displacement of 1.75″ based on my sons actions is applied to the outer most portion of the large ring.

Due to large displacement in the model and the nonlinearity of the material Simulation Premium was used to analyze the geometry. Note the material specifications are not exact and a generalized S-N curve was used for the fatigue analysis. A zero based loading was used for fatigue.

Results:

Original Design Showed 20000 PSI of stress at the fillet area.

A fatigue life of 890 cycles for the same area.

Stress Large Fillet

 

 

 

 

Original Design Fatigue

The modified larger fillet showed much improvment Stress of 10000 PSI and a fatigue life of 4000 cycles.

Stress Original Design

Fatigue Modified Design

If the geometry is modified with a larger fillet the toy will be enjoyed a great deal longer.  From the design change a life of 4000 cycles, my son will be 4.  Before then, I am sure it will be passed on to a little brother or sister.

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Posted in COSMOSWorks, Simulation, Uncategorized | 3 Comments »

Divergence and Convergence for Simulation Results

Friday, April 22nd, 2011

Divergence in a model  leads to a stress quantity that continually increases as the user refines the mesh globally or in areas of localized high stress.  Divergence is typically encountered when the load transfer concentrates in the geometry at a sharp corner. The stress will continue to rise based on its own definition;

Stress=Force/Area

With a sharp corner the area becomes, zero.  This drives the stress value upwards with each new, tighter mesh. In manufacturing a perfect edge does not exist. All modern manufacturing methods will create small fillets.  The fillet helps transfer the load over a larger area hence eliminating the stress concentration.

4-22-2011 8-49-13 AM

Lets Discuss Convergence In a Simulation Model:

In Finite Element Analysis (FEA) the results are an approximation.  The way a user becomes comfortable with the stress, strain, and deflection calculated by an analysis depends on convergence.  A recommended variation of 5% stress between runs usually indicates good stress convergence.

To reach convergence a user can apply multiple methods, over the course of their analysis studies.

Global Mesh Refinement

The Global mesh refinement method indicates that a user over the course of three or four studies tightens the global mesh size incrementally until the calculated results are within the users specified convergence range.

    Plus: Easy to adjust the setting of global mesh size.

Minus: Larger run times due to areas of low stress having the mesh refined as well.  The more elements in the Finite Element Model the longer the computing time.Mesh Control

Mesh control allows a user to specify mesh refinement in localized areas of high stress leaving the global mesh density larger.  This is the recommended method.

    Plus: Allows user more control over the mesh in areas of high stress. Keeps computation time shorter as only areas of interest are being refined.

Minus: This takes more time to setup and identify areas to add mesh control.Adaptive Meshing (H Method)

Adaptive meshing allows the users to apply “mesh cruise control”.  Simulation looks for convergence based on parameters the user specifies, usually a target accuracy for the Strain Energy Norm.  Simulation automatically adds mesh control in areas of calculated high stress. It is important that the original mesh starts at default settings, and it is recommended this method be used with standard meshing. Five iterations is usually a good starting point.  Note that H Adaptive is only run once, as the software completes the mesh refinement and runs for you.  Remember that if the target accuracy is not achieved, you can re-run the H-adaptive study again and it will continue from the last iteration.

    Plus: Simulation automates the process.

Minus: Additional analysis setup time is required.When adding mesh control or mesh refinement always take small steps.  Mesh density should be adjusted in small increments. For example if a mesh control starts at 0.25″ the next change would be 0.2″.  We generally recommend a 20% reduction in mesh size with each successive analysis for applied mesh controls.

Let’s discuss Standard versus Curvature based mesh:

The Standard Mesh has been used from the beginning of SolidWorks Simulation.  The Standard mesher uses the Voronoi-Delaunay meshing scheme for subsequent meshing operations. The Standard mesher incorporates a global size that reads the thinnest area of the component and dictates at minimum cross section at least two elements define the thickness in the mesh.  This may lead to mesh refinement  in areas that may not be required.  Manual mesh control can then be applied to refine specified areas.

The Curvature based mesh creates more elements in higher-curvature areas automatically (without the need to apply mesh controls).  Typically curvature based meshes do not need additional user applied mesh controls, and take less steps to achieve convergence.

Mesh Quality

The output from any simulation is mesh dependent.  An analysis of a model with a  good quality mesh will have different answers than the same model with a poor quality mesh. It is always important to look at the mesh plot and investigate the aspect ratio of the mesh.  Aspect ratio is a measure of the skew of an element, defined as the ratio of the longest edge/ shortest edge.  For a high Quality element, the Aspect Ratio should be less than 3.  Overall 85% to 90% of the elements in a mesh should have an Aspect Ratio of less than 3. A high Aspect Ratio will cause higher stress to be calculated in that element than surrounding cells.

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Posted in COSMOSWorks, Simulation, SolidWorks 3D CAD, Tips & Tricks, Uncategorized | 1 Comment »

Simulation Motion Helps With Difficult Billiard Shots

Wednesday, March 30th, 2011

Several engineers here at 3DVision got together recently and played billiards.  While expressing our favor towards the game we discussed the mathematics and mechanics involved.  No matter how you look at it, Geometry and Physics rule the billiard table.

Which brings me to Simulation Motion, the attempt here is to accurately predict tough billiard shots using the Motion Analysis package.

This blog details the first attempt at Simulating a billiard shot. The model was created to gain information for future Motion Analysis; the model is comprised of a billiard table, two billiard balls, and a cue.

3DVision Billiard Table

The billiard table is a multi-body part allowing for different material properties depicting the slate bed and rubber rails.

The billiard balls are either composed of Bakelite or Aramith.  Bakelite is the custom material in billiard balls in the 1970s; its polymer blend has a wealth of material information specifically for this application.   The newest material, Aramith, has replaced Bakelite since the 1990′s.  However, there is little information on its material specs.   Looking into the playability between the two materials, credible sources say there is no difference between the billiard balls.  In this model Bakelite is the best material, and is used for the billiard balls.

The cue is Rock Maple with a rubber tip.  (Rubber is used instead of a layered cowhide tip due to lack of material specifications on leather.)

Contact conditions were specified between the table rails and the billiard balls, billiard balls and table bed, billiard ball to billiard ball, and cue ball to cue stick.

A displacement was assigned for the cue acting through 3″ of movement in 0.25 seconds.

Motion accurately simulated a basic cut shot.  This is a preliminary setup using assumed friction coefficients and material properties.  This trial run demonstrates the setup of more advanced real world billiard shots including: adding English and draw to the cue ball to allow for proper cue ball placement for subsequent shots.

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With more realistic boundary conditions and Simulation Motion, I will attempt to simulate the tougher professional billiard shots.  As well as detect and report the stress being generated by the impact of the cue to the cue ball, and billiard ball to billiard ball collisions.  Stay tuned for more information and new shot videos.

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