Gone Skiing: Aerodynamics – Does It Matter Which Way Your Skis Are Pointing On Your Roof Rack?

I was on the gondola up at Keystone for night-skiing a week ago, after a long day at Beaver Creek, because the last thing I am going to do at 3:00 pm is try to make it back to Denver, as everyone knows it’s hardly more than a parking lot at that point. As it gets later, there’s nothing like a solo gondola ride, however, a solo ride would stop this story right about now.

On the gondola, I overheard a conversation where one gentleman was discussing how he was unable to open the hatch of his vehicle when his skis are in his roof rack. That’s fair, I know older WRX wagons with the spoiler would not be able to open with skis on the roof no matter what, so I figured that was the case. It turns out, that was NOT the case. The reason his hatch would not open was that he orients the skis with the tails forward because it is ‘more aerodynamic’ that way… I was skeptical, but held my tongue, knowing that I had the tools at my disposal to investigate!

I decided to make a model that would allow me to simulate various conditions to get to the bottom of this. My initial hypothesis is that the addition of the ski rack and crossbars is what has the largest effect on aerodynamics, and orientation of the skis probably has a negligible effect after that. As a side note, I am solely concerned with aerodynamics in this case, and am not worrying about the amount of the ski’s base material that is exposed for a given orientation. I am of the mindset that tree trunks and hidden rocks on the mountain are more of a danger to your bases than small rocks on the highway anyway. If you are waiting to comment, “Just get a roof box!”, I understand as I own both a box and a rack at this point, and they both have their advantages, and I will not be exploring the aerodynamics of a box…

…yet…

I was able to start by finding some faceted geometry of a Subaru Forester online (I’m from Colorado, can you tell?) and was able to import that into ANSYS Spaceclaim. Once in Spaceclaim, I was able to edit the faceted geometry to get nice exterior panel surfaces, which I then combined to get a single clean faceted exterior for the car.


Faceted Forester Geometry (Equipped with factory side rails)

After that, I used Spaceclaim to generate the remainder of the rack and skis, including crossbars, a ski rack, and a pair of skis (Complete with the most detailed bindings you have ever seen!). I made a combined part of the crossbars, rack, and skis for each one of my orientations, as this allows me to report the forces on each combined part during the simulation.


Added CAD geometry for the crossbars, ski rack, and a pair of skis

For the simulation, I used ANSYS Discovery Live, the newest tool from ANSYS that allows for instant and interactive design exploration. This tool lets me actively add my CAD geometry and shows results in realtime. I was able to start with just the car and then add and swap my ski/rack geometry with simple button clicks. With traditional simulation tools, I would have needed to create a mesh for each one of these cases, analyze them one at a time, and the post-process and compare results after the fact. After launching Discover Live, it’s as easy as selecting the type on analysis I want to run.

The various types of solutions that can be done in ANSYS Discovery Live. For the purpose of this blog, I am using ‘Wind Tunnel’

Once I have selected ‘Wind Tunnel’ for my solution, I can select my geometry, and then am prompted for the direction of flow, as well as selecting the ‘floor’ of my domain. Once that is done, results show up on the screen instantly. I only needed to modify the flow velocity to ~65 mph. I am most interested in the force on the faces of the combined crossbars, rack, and skis in each orientation, so I created Calculations for each one, which is done by simply selecting the part and using the popup toolbar to create the graph.

Popup toolbar allows for the quick creation of solution calculations

I was already off and running. I ran each one of the cases until the force plot had become steady.

Car Only
Skis Tips Forward Orientation

Skis Tails Forward Orientation

Seeing that the force results for the Tips Forward vs. Tails Forward cases were very similar, I decided I should also run a ‘Bases Up’ Orientation, even though I STRONGLY advise against this, as UV wrecks the base material of your skis/snowboard.

Ski Bases Up and Tips Forward Orientation

In addition to the contour plot shown in the images above, you can also use emitters to show streamlines and particle flow, which also give some pretty neat visualizations.

Streamlines shown on the Tips Forward orientation

Particle Emitter shown on the Tips Forward orientation

The graph plots show values for the Total Y Force for Tips Foward, Tails Forward, and Bases Up orientations to be 37.7 N, 39.1 N, and 37.1 N, respectively. Using Discovery Live, I was able to quickly run all 3 of these simulations, showing that there is not a major difference in the forces on the ski rack between the three orientations. So, put the skis on the roof in the direction that makes life easiest for you, and keep those bad boys paired to protect your bases from the sun, because splitting them isn’t going to help with aerodynamics anyway!

Next steps would be taking a specific case and running in 2D, then 3D, in ANSYS Fluent.

Explore the fluid flow capabilities available in Discovery Live – Webinar

This free webinar will cover:

  • How to gain early insights into trends of pressure drag, drop & drift
  • The capability of gathering qualitative information for flow dispersion & thermal mixing
  • Exploration of recirculation zones and experimentation with changing flow direction
  • And so much more!

Don’t miss this informative presentation – Secure your spot today!

Can’t make it? Register anyway, as a recording will be made available to be watched on-demand.

If this is your first time registering for one of our Bright Talk webinars, simply click the link and fill out the attached form. We promise that the information you provide will only be shared with those promoting the event (PADT).

You will only have to do this once! For all future webinars, you can simply click the link, add the reminder to your calendar and you’re good to go!

All Things ANSYS Episode 008 – Looking at Explicit Dynamics and CFD tools in ANSYS pt. 1

Published on: November 6, 2017
With: Tom Chadwick, Joe Woodward, Jim Peters, Ahmed Fayad, Eric Miller
Description: In this episode your host and Co-Founder of PADT, Eric Miller is joined by PADT’s Senior CFD Engineer Tom Chadwick, Senior Mechanical Engineer Joe Woodward, Senior Staff Technologist Jim Peters, and IT Operations and Support Engineer Ahmed Fayad for the first part of an in depth look at Explicit Dynamics in ANSYS along with a review of the various CFD tools available in the ANSYS family of products.
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Introducing: The PADT Startup Spotlight

In support of the ANSYS Startup Program, PADT is proud to introduce the PADT Startup Spotlight.

We here at PADT are firm believers in the opinion that today’s startup companies are tomorrow’s industry leaders and thus should be give every possible opportunity to thrive and succeed.

As a result we are offering full access to our promotional capabilities in order to help startup companies developing physical prototypes to grow and develop in a competitive environment.

We will look through those startups that have purchased the ANSYS Startup Package through PADT, and select one to feature and promote, that we believe clearly represents the drive and entrepreneurial spirit that is key in order to succeed in today’s day and age.

Presenting the first Startup Spotlight:

Since their inception in 2014, Velox Motorsports has always been focused on speed; whether that be the speed of the NASCAR teams they have worked with or the desire their customers have for speed, which drives their competitiveness and fuels the demand for their products.

They even show a passion for speed in the company’s name (Velox), which translates from Latin to “swift or speed”.

Visit www.padtinc.com/startupspotlight for more information on Velox Motorsports and The PADT Startup Spotlight.

Active Solution Monitoring during a solve in ANSYS CFX

One of the cool new features in CFX 18 is the ability to actively review results while the calculation is running. It is supported for steady state and transient calculations, and now includes support for rotating reference frames as well.

What follows are some tutorial-esque steps to get you started.

crm_10275-active-solution-monitoring-CFDPostR18