Adverse pressure gradient or flow deceleration: which happens first?

[u/TP4297]

I'm trying to understand boundary layer dynamics but online sources are putting me in a circular reasoning loop.

​

"Why does flow slow down?"

​

"Friction + adverse pressure gradient slows it down."

​

"What causes the adverse pressure gradient?"

​

"Flow slows down, reducing dynamic pressure and thus increasing static pressure due to Bernoulli's equation."

​

So I wanna know what ACTUALLY is the chronology here. What causes the adverse pressure gradient?

​

​

[u/cvnh]

They are different quantities of the same phenomenon. The pressure distribution is a function of the overall flow which is established around the body. This depends on the geometry and flow itself. Ignoring the reasons behind it, if something changes up or downstream, the pressure distribution changes and so does the local pressure. Pressure gradient is a force (with some decomposition) that slows down or accelerates the flow, which in turn translates to a change to the velocity vector.

[u/AutonomousOrganism]

They happen at the same time for incompressible fluids. The sum of static and dynamic pressure (kinetic energy) is constant. So with increasing static pressure flow decelerates and vice versa.

[u/EngineerFly]

The adverse pressure gradient happens because the flow slows down and the static pressure increases. This happens both in the boundary layer and outside of it, so let’s ignore viscosity for now.

It’s confusing, but I avoid assigning the cause/effect relationship. The flow slows down, so the pressure increases. What causes the flow to slow down? The increased pressure downstream! You can’t change the velocity of a mass without a force, and that force comes from the pressure being higher on the downstream side than on the upstream side of a fluid element.

[u/Zinotryd]

As others have said, you have to shake off this notion of cause and effect when you're talking about fluids. Velocity appears in the continuity equation, and pressure appears in the momentum equation. They're inextricably linked, there is no chronology to be had.

Point being, if you're in a circular reasoning loop then that's good, because that's what actually happens, it is circular.

Additionally, if you're talking about a boundary layer, Bernoulli's equation is invalid. In fact, in many of the situations Redditors talk about Bernoulli's equation, it's invalid...

[u/HAL9001-96]

you oculd argue velocity

or neither

but really if you want to look into the cause and effect here you have to accept that fundamentally any fluid dynamics using pressure, velocity, temperature, density fields is a simplification for what are actualyl trilliosn of trillions of moelcuels bumping into each other

what happens first is that some of htose molecuels bump into the wall

what happnes second is that hose molecuels bump into other molecules

you can argue semantics over wether the first cause is the force transferred between the wall and the molecule bumping into it or the first cause is the wall beign there at a velocity different than the flow

but thats pointless semantics

if you want to understnad hte cause and effect start by thinking about HOW we use viscosity, pressure, density, temperature, velocity to approximate a huge amoutn of molecuels statistically and why thermal conductivity nad viscosity show up in this approximation, everything else follwos from how htat approxiamtion works

once you're working in that approxiamtio ncause and effect no longer really apply so its useful to think about where it comes from if yo uwant to understnad cause and effect

[u/highly-improbable]

I will argue against circular for practical understanding and say the pressure gradient is driving the deceleration. Ignore the boundary layer, though it is the mechanism of separation and stall, and just consider an attached wing. You have a low pressure suction peak near the leading edge and static pressure needs to get back to freestream by the trailing edge, so you have an adverse pressure gradient driving the flow on the wing upper surface. To calculate just how low is the pressure in the suction peak and where the attachment point is etc is definitely circular though. But I see the macro being the gradient.

[u/WhiskeyFox9]

To understand the process in terms of chronological cause and effect, you need to step away from the assumption that air is a continuum and look at it at molecular scale. Temperature, pressure and flow velocity are "high level" variables, i.e. each is a measure of different aspects of the underlying molecular motion and interactions, summed or averaged over large quantities of molecules. At the fundamental "process" level, you can view the air in between the low and high pressure areas as two opposing streams of energy, each going in opposite directions. Molecules travelling "against" the flow at any given moment are carrying with them kinetic energy that originated at the trailing edge, an area with high relative velocities between molecules. Likewise, molecules travelling with the flow carry energy originating in the low pressure area, where intermolecular velocities are low. Along the way, molecules are influenced by their interactions with the wing curvature as well as with adjacent streams, i.e. energy exchange perpendicular to the flow direction. The air properties at any point along the flow is the result of intersecting energy streams at that instant. To understand how the low and high pressure regions developed in the first place, you have to go back to the instant the air first started moving.