# Does flow separation starts earlier at higher airflow speeds?

For same AoA,will flow separation starts earlier (point of separation more upstream) at higher airflow speeds (=higher air inertia) compare to lower airflow speeds?

High speed airflow has more air inertia so it seems inuitevly that air will harderd follow curved surface and leave surface earlier?

Question referes to subsonics speeds,but you can expand your answer at supersonic speeds as well.

Same topic at physics site,but we heve problem here because members have opposite oppinions: https://physics.stackexchange.com/questions/602876/does-flow-separation-starts-earlier-at-higher-airflow-speeds/602888?noredirect=1#comment1356599_602888

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• We are taught as young pups that stall is AOA specific, independent of speed, so the answer should be no, but I'll leave it to someone who can express it in proper fluid dynamics terms. Dec 25, 2020 at 6:41
– user53913
Dec 25, 2020 at 6:50
• @ebv821 John K's statement is generally correct within the speed range a given plane flies. Reynolds effects are over orders of magnitude, and can be seen on (Airfoil Tools) polar diagrams. Try the Clark Y for starters. Dec 25, 2020 at 12:41

It is generally assumed in incompressible aerodynamics that the Reynolds number over a wing is so high that the effect of the Reynolds number (same air viscosity, same wing chord, only variable in Re is speed) on the lift coefficient is negligibly small. I can think of two areas where this assumption breaks down:

• Very small (for example RC model scale) wings, where the Reynolds number is appreciably reduced. As the air in this case is less turbulent, flow separation occurs earlier.
• Compressibility effects such as shockwaves occuring on top of the wing at transonic speeds, where we have to include not Reynolds but Mach number in our lift coefficient estimation. The effect of Mach number on max. lift coefficient depends on the wing design, some fighters are optimized for it, while most aircraft have their max lift coefficient and max angle of attack reduced at transonic speeds.
• If you want correct results with small model scale ,then you must increase pressure or decrease viscosity to match same Re number as full size object.You cant increase speed because Mach number will not be the same.But why you will use reduced Re number compare to full size object?
– user53913
Dec 25, 2020 at 21:20
• I meant RC models, now clarified Dec 25, 2020 at 21:34
• Ok but you did not write answer for topic question,does high speed make earlier or delayed separation..?
– user53913
Dec 25, 2020 at 22:10
• "...the effect of Reynolds number on the lift coefficient is negligibly small..." The only thing that can effect Re is speed. Lift coefficient may be that of any condition, but we're interested in maximum lift coefficient, so the effect of speed on maximum lift coefficient is negligibly small, it does not delay or advance separation, bar the two cases I have mentioned. Dec 25, 2020 at 22:13
• At this graph higher Re numbers strictly means higher speeds?How this is posssible if I can get higher Re numbers if I increase density or decrease viscosity etc. I dont know is Re=10 honey or air, if I can make so low Re numbers even with air If I reduce speed or length,or increase viscosity etc lh3.googleusercontent.com/proxy/…
– user53913
Dec 25, 2020 at 23:04

A quick study of the Clark Y airfoil polars shows stall angle of attack increases as Reynolds number increases.

high speed airflow has more inertia, so it seems intuitively it will be harder to follow a curved surface

We can observe that the area of compressed air in front of the wing forces the free stream to bend earlier at higher velocity. This means, inertially the free stream will follow less of a curve (and the transition zone starts further forward).

At higher airspeeds the flow separation point also moves back, creating a larger, more stable area of lifting low pressure above the wing. Notice, at lower airspeeds, many planes employ vortex generators to energize the airflow in an effort to squeeze a few more degrees of AOA before separated airflow collapses the lift bubble, creating a stall.

What makes this study a little confusing is the effect of velocity on lift. Most aircraft lower AOA at higher velocity long before a change in Reynolds number effects the separation point.

Hence, we universally are taught that AOA is the critical factor in "stall" flow separation regardless of airspeed (within the planes normal flight envelope).