Predicting airflow pattern over a body
 

What I am looking for is websites and articles, not 3 page long explanations here- although they are welcomed if someone is feeling up to explaining what I am about to ask right here on this forum. That would be very much appreciated. I just bought a 120 dollar book and I am NOT buying new books on aerodynamics anytime soon because before I plow through 1000 pages of solid calculus and trigonometry... well that won't happen soon for sure.

My question is, how can you predict the pattern of airflow over an airfoil or a ramp. How can you rpedict where the boundary layer will detach itself from the airfoil surface? Is there a formula or a function which will allow me to know how far along from the leading edge will the boundary layer go turbulent over my wing?

Next question. Say that I have some big block sticking out of the side of my airplane. I want that block to remain there but I don't want the air to encounter it face to face. So, I put a ramp or some smooth surface in front of that block so that the air flows around it without particularly meeting the block face to face. I want to know how big or steep that ramp should be so that the air just flows past the block- not too far away, not to close to it.

These are all jsut concepts and I don't have an airplane with a block sticking out of it, so please dont tell me that a picture would help. Thanks :D.

I appreciate answers and links very much, thanks guys.

Lafayette.

RE: Predicting airflow pattern over a body
 

It's darn near impossible to accurately tell when the air will separate other than with direct flight testing or in a wind tunnel. The problem is that the angle of attack where this happens is so strongly tied to the reynolds number and the surface finish. Having said this a lot of the work being done in new airfoil designs by Selig, Drela and many others is geared to achieving shapes that show smooth and not overly steep pressure distribution curves when you run the shape through something like Xfoil or the Eppler predictive software. My favourite way of accessing Xfoil is through Profili2 which comes with it embedded and uses a far nicer interface than the command line version. To unlock the Xfoil and other features in Profili2 costs something like $20US paid to the writer. Or if you are stubborn you can download Xfoil for free and learn to use it from the command line. There's also an online forum for Xfoil to discuss all the ins and outs.

For shapes that have some serious discontinuities in the pressure distribution curves you can often figure on the airflow separating at or near the location where there's a radical bend in the curves. But even then it's dependent to a big extent on the Reynolds number and surface finish. Rougher finishes promote a turbulent boundry layer which tends to hold the airflow tighter to the wing while smooth and accurate shapes promote laminar flow for a longer portion of the airflow. The deal is that laminar flow can detach from the surface moer easily.

One place where you can be assured that you'll get a separation bubble is on a D tube style wing with the sheeting ending at the high point. When you cover this design the covering shrinks and pulls tight forming a really nice sharp discontinuity in the curve right at one of the worst possible points At any sort of semi seroius lift task for the wing you'll find at least a little sepration bubble between each rib and just downstream from this ridge.

As for the front fairing you'd want it to be shaped like the front end of a big thick symetrical airfoil where the curve just comes to the point where the last bits are parallel and in line with the sides of the block. The fairing block being airfoiled like this in a squarish shape and in two directions. Ideally it would use the back half of the airfoil behind the block to ease the air back together.

As for a more accurate modeling of the airflow it comes down to a few very serious computational flow dynamics programs. Us mere mortals can't even begin to think of what this costs.

RE: Predicting airflow pattern over a body
 

First, let me say that a picture would help. I don't know what you mean by a 'ramp', and a picture of what you are considering would help clear that up. Generally, you want a fairing around the block, rounded on the leading edge and gradually tapering to the fuselage on the trailing edge. Don't tell me that you want a picture. Think about the shape of wheel pants.

As Bruce recommended, Xfoil is probably about the best you will do for predicting where separation or transition to turbulence will occur, under a particular set of conditions, for a shape like an airfoil. For something other than an airfoil, like a block, sharp edges are always likely candidates for separation points.

banktoturn

RE: Predicting airflow pattern over a body
 

Lafayette,

Have to agree with BMathews. If you're playing with a new design with many unknown variables. There is no accurate mathematical way you can predict air movement within a three-dimensional space without spending a LOT of money. Think of it, even aircraft companies, Formula one racing teams, and car companies STILL use wind tunnels. Even they need help from a physical model. Not only do you have to solve for R, surface finish, etc, you will also need to solve for airflow that goes around the object at all angles since you don't have an infinitely long ramp, sideslip, airflow around the base where the ramp, etc. ...and that's just the beginning of your problems. Now solve for 3 dimensions.

One 'cheat' would be to use something like XFOIL or a really old version of SNACK. Pick a likely airfoil candidate and look for separation. Now make a model and test it on a wind or water tunnel.

Or, you can plow through 1000 pages of calc and trig. I did. BTW, on your other post with the picture of the formulas, if the formula is for this particular problem, it's the wrong one.