How does surface irregularity affect boundary layers?

[underflow21]

TL;DR Summary

can anyone explain the physics behind surface roughness affecting boundary layers and flow separation?

I was reading an article and it mentioned that surface roughness can affect boundary layer transition? How? I am just a casual reader of aerodynamics but have a background in engineering can anyone explain the physics behind surface roughness affecting boundary layers and flow separation? and also, how would, say a gouge have an effect on boundary layers? I read an article from the u.s airforce:

'Surface Irregularity Effects on Laminar-Turbulent Transition - understanding the surface quality requirements for laminar flow on wings'

It mentioned rossiter and t-s waves but it didn't really make sense to me. if anyone could break it down, would be amazing

thanks y'all

 

[Lnewqban]

Welcome, @underflow21 !

Surface roughness (or contamination with ice and insects), combined with Reynolds number, affect the boundary layer transition, by creating a sudden negative or unfavorable pressure gradient in the air flowing over the surface.

Copied from this old but well detailed NASA paper:
https://ntrs.nasa.gov/api/citations/19980232017/downloads/19980232017.pdf

"The impact of a surface imperfection (such as a rivet head) on the transition location can be viewed either by looking at the transition location as a function of imperfection size for a fixed unit Reynolds number or by keeping the size of the imperfection fixed and looking at transition location as a function of unit Reynolds number. The illustration in figure 17(Holmes et al. 1985) depicts the latter case, where the amount of laminar flow is decreased as Reynolds number is increased. The problem is then to determine what roughness height and shape for a given Reynolds number will cause a reduction in the amount of laminar flow obtainable. In either case, the imperfection stimulates eigenmodes in the boundary layer; the linear stability of the flow dictates whether these modes will grow or decay as they evolve in the flow. However, as the height of the imperfection or unit Reynolds number increases, a point is reached when flow separation occurs because of the surface imperfection."

 

[Lnewqban]

Copied from
https://en.wikipedia.org/wiki/Reynolds_number

"The Reynolds number is the ratio of inertial forces to viscous forces within a fluid that is subjected to relative internal movement due to different fluid velocities. A region where these forces change behavior is known as a boundary layer, such as the bounding surface in the interior of a pipe. A similar effect is created by the introduction of a stream of high-velocity fluid into a low-velocity fluid, such as the hot gases emitted from a flame in air. This relative movement generates fluid friction, which is a factor in developing turbulent flow. Counteracting this effect is the viscosity of the fluid, which tends to inhibit turbulence. The Reynolds number quantifies the relative importance of these two types of forces for given flow conditions and is a guide to when turbulent flow will occur in a particular situation.
...
With respect to laminar and turbulent flow regimes:

• laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion;
• turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities."

The Reynolds number is defined as:

 

[jack action]

When the laminar flow breaks down, the fluid begins to tumble down on the surface of the object.

By having a rougher surface, these small disturbances tumbling down tend to bring the flow back to the surface. (Imagine the fluid being over a sticky rolling barrel and following its surface as it rolls down.)

Finding the sweet spot between higher roughness and large disruptions, you can extend further the distance where the flow will separate and become fully turbulent.

The best example is the golf ball with and without dimples:

 

[boneh3ad]

Actually, except in the case of very large surface irregularity, the effect has little or nothing to do with pressure gradient.

The full answer here is rather complex and still an active area of study. It also depends on the nature of the roughness in question. Isolated roughness is different from distributed random roughness, which is different from periodic roughness.

The short answer is that boundary layers transition due to the growth (and eventual breakdown) or unstable waves that propagate through them. The transition location is determined, among other things, by the initial amplitude of these waves. The initial amplitude of the waves is determined by a process called receptivity through which unsteady disturbances in the free stream interact with a surface to produce unsteady disturbances in the boundary layer. One of the primary mechanisms for this receptivity process is the interaction of surface roughness with free-stream disturbances to produce waves that can grow and eventually transition to turbulence. [1]

In essence, larger roughness produces larger initial amplitudes for the waves that are then subject to boundary-layer instability. That means they reach a critical amplitude and transition sooner.

This is why dimples on golf balls produces turbulent flow sooner than a smooth ball, which resists separation, producing a smaller wake and lowering overall drag (even though viscous drag increases).

References:
[1] Saric, WS, Reed, HL, Kerschen, EJ. 2002. "Boundary-Layer Receptivity to Freestream Disturbances." Annual Review of Fluid Mechanics. 34. pp. 291-319. doi: 10.1146/annurev.fluid.34.082701.161921

 

[underflow21]

Actually, except in the case of very large surface irregularity, the effect has little or nothing to do with pressure gradient.

The full answer here is rather complex and still an active area of study. It also depends on the nature of the roughness in question. Isolated roughness is different from distributed random roughness, which is different from periodic roughness.

The short answer is that boundary layers transition due to the growth (and eventual breakdown) or unstable waves that propagate through them. The transition location is determined, among other things, by the initial amplitude of these waves. The initial amplitude of the waves is determined by a process called receptivity through which unsteady disturbances in the free stream interact with a surface to produce unsteady disturbances in the boundary layer. One of the primary mechanisms for this receptivity process is the interaction of surface roughness with free-stream disturbances to produce waves that can grow and eventually transition to turbulence. [1]

In essence, larger roughness produces larger initial amplitudes for the waves that are then subject to boundary-layer instability. That means they reach a critical amplitude and transition sooner.

This is why dimples on golf balls produces turbulent flow sooner than a smooth ball, which resists separation, producing a smaller wake and lowering overall drag (even though viscous drag increases).

References:
[1] Saric, WS, Reed, HL, Kerschen, EJ. 2002. "Boundary-Layer Receptivity to Freestream Disturbances." Annual Review of Fluid Mechanics. 34. pp. 291-319. doi: 10.1146/annurev.fluid.34.082701.161921

I was thinking more of insect buildup on airplane wings, surely the buildup, even if randomly distributed would have an effect, no, was thinking that it could possibly trigger an earlier transition.

If I understand it correctly from what you said, the disturbances to the surface could trigger earlier transition due to the increased amplitude of the waves?

 

[boneh3ad]

I was thinking more of insect buildup on airplane wings, surely the buildup, even if randomly distributed would have an effect, no, was thinking that it could possibly trigger an earlier transition.

If I understand it correctly from what you said, the disturbances to the surface could trigger earlier transition due to the increased amplitude of the waves?

Insect buildup is definitely an issue (though more for wind turbine blades than for aircraft). Initially it basically adds surface roughness. A few insects here and there act like isolated roughness that can generate local turbulent wedges. Lots of insects would look more like distributed random roughness that can cause early transition across the span. If you get enough you could start to meaningfully alter the OML of the airfoil similar to what happens with icing.

 

[underflow21]

Insect buildup is definitely an issue (though more for wind turbine blades than for aircraft). Initially it basically adds surface roughness. A few insects here and there act like isolated roughness that can generate local turbulent wedges. Lots of insects would look more like distributed random roughness that can cause early transition across the span. If you get enough you could start to meaningfully alter the OML of the airfoil similar to what happens with icing.

is the effect similar if it was negative surface roughness? like if you had a dimple/dent/chip in a wing what would happen to the boundary layer, would it be similar to the golf ball example? because a wing is obviously not spinning, so can't see how that effect would also work on wings. would there be more recirculation due to that or more vortices being created
also thank you so much for answering me, I have kind of gone down a rabbit hole and I'm finding all of this quite fascinating

 

[Lnewqban]

... so can't see how that effect would also work on wings. would there be more recirculation due to that or more vortices being created
... I'm finding all of this quite fascinating

It is fascinating!

Once the wing is dealing with turbulent flow, the next goal is to prevent or delay flow separation.

When the wing is lifting heavy loads, and flying relatively slow, and at a high angle of attack, it becomes harder for the airflow to "suck" on its top surface.

That is where turbulence is encouraged by several means, which add mechanical energy to the "sucking" airflow.

Please, see:
https://en.wikipedia.org/wiki/Flow_separation

https://en.wikipedia.org/wiki/Vortex_generator

 

[boneh3ad]

Negative surface roughness can be similar as long as it isn't too large. Both would enhance transition and could, if large enough, generate turbulent wedges immediately downstream. Once the amplitude of the roughness gets large enough, you start seeing other discrepancies. For example, blemishes going into a surface, when they get deep enough, can lead to phenomena like Rossiter modes and similar situations where the flow setting up inside the cavity interacts with the flow going over it. But there is also a certain depth where making it deeper has no real further effect on the external flow.

For large positive roughness, if you keep making it bigger, the effect it generates keeps getting bigger. There is no saturation effect like you would see with negative roughness.

Some of the idea behind golf ball dimples absolutely works on wings. No rotation necessary. Roughness on a wing will encourage turbulent flow. Turbulent flow is more resistant to separation, so you are less likely to get separation and stall on a wing if the flow is turbulent. However, a far smaller percentage of drag on a wing is due to the size of the separated wake when compared to a sphere, so delaying separation has very little impact on drag on an airfoil in most practical cases. Therefore, causing transition on a wing usually makes total drag go up, not down, because of the dramatic increase in viscous drag. You would therefore usually rather have a laminar wing during steady flight (cruise).

Sometimes it's better to have turbulent flow, though, such as when performing maneuvers requiring high angle of attack or during takeoff and landing. In those situations, the wing is often operating much closer to stall so having turbulent flow will help the boundary layer resist that.

 

[underflow21]

It is fascinating!

Once the wing is dealing with turbulent flow, the next goal is to prevent or delay flow separation.

When the wing is lifting heavy loads, and flying relatively slow, and at a high angle of attack, it becomes harder for the airflow to "suck" on its top surface.

That is where turbulence is encouraged by several means, which add mechanical energy to the "sucking" airflow.

Please, see:
https://en.wikipedia.org/wiki/Flow_separation

https://en.wikipedia.org/wiki/Vortex_generator

thank you so much for the replies guys, it's all really appreciated, I feel like I'm back in college

 

[boneh3ad]

thank you so much for the replies guys, it's all really appreciated, I feel like I'm back in college

Hopefully that's a positive thing?

 

[underflow21]

Hopefully that's a positive thing?

haha yes, I have not been to college in a looong time but I loved it, I am thinking of going back and maybe getting a masters because I miss learning.

 

[Tom.G]

haha yes, I have not been to college in a looong time but I loved it, I am thinking of going back and maybe getting a masters because I miss learning.

Naww, save your money, just keep posting here!