What causes pressure in form drag?

Click For Summary

Discussion Overview

The discussion centers on the causes of pressure in form drag, particularly in the context of a ball flying through the air. Participants explore the effects of turbulence, flow separation, and wake size on pressure distribution around the object, with a focus on understanding the pressure dynamics at the rear of the object.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes how turbulence and flow separation on a smooth ball lead to circulation in the wake, resulting in pressure differences that contribute to form drag.
  • Another participant questions why pressure increases at the rear of the ball when there is less circulation, despite a smaller wake, seeking clarification on the underlying mechanisms.
  • Participants discuss the applicability of Bernoulli's equation, with some suggesting it may not apply due to the viscous nature of the flow, while others argue that it can still be relevant under certain conditions.
  • There is mention of energy loss affecting the full extent of Bernoulli's principle, indicating a nuanced view of its applicability in fluid dynamics.
  • One participant references external materials and articles, indicating a search for more comprehensive explanations regarding the pressure dynamics involved.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Bernoulli's equation in the context of viscous flow, and there is no consensus on the reasons for pressure changes at the rear of the ball. The discussion remains unresolved regarding the specific mechanisms that lead to increased pressure in the absence of circulation.

Contextual Notes

Participants acknowledge the complexity of the topic, including the influence of viscosity and wake size on pressure dynamics, but do not resolve these complexities within the discussion.

JTC
Messages
100
Reaction score
6
Consider a ball flying through the air.

When there is turbulence, and the flow separates, say on a SMOOTH ball, then in the rear, there is circulation in the wide wake. There is pressure on the front, but no pressure on the rear due to the fact that the fluid is "busy" circulating around. So there is form drag.

When there is NO turbulence (or little) and the flow does not separate as early, say on a golf ball, then there is LESS circulation in the rear in the smaller wake.

So my question is: What gives rise to the pressure in the rear, when there is an "annealed" or "healed" flow (when the dimples on the golf ball make the wake smaller).

Where is the higher pressure coming from in the rear to match the pressure at the front end?

(I am not asking about the dimples on golf balls. I am only asking why the pressure goes up in the rear. Yes, I know there is a smaller wake but that does not explain why the pressure goes up.)
 
Last edited:
Engineering news on Phys.org
BvU said:
https://www.scientificamerican.com/article/how-do-dimples-in-golf-ba/

In other words... Why is there lower pressure in the circulation? I would guess Bernoulli's equation in in the slower flow, but Bernoulli should not apply
do to the viscous nature.

And why is the pressure higher without the circulation?

The article falls flat right where I want the answer.
 
JTC said:
Bernoulli should not apply due to the viscous nature.
are you saying Bernoulli does not apply to air ?
 
BvU said:
are you saying Bernoulli does not apply to air ?

Well, this is where all the confusion for me is.

I would think that no, it does not. But if one is studying effects induced by viscosity, then Bernoulli cannot be used to justify something. I think.
 
Bernoulli definitely applies (see story about truck and car being pushed towards each other), but not the full extent since there is energy loss.

When I read the Jestocott link or this link, the emphasis is on the size of the wake. But you stll need the pressure in the wake to be lower in order to get that effect of bigger wake ##\Leftrightarrow## more drag.
 
  • Like
Likes   Reactions: Lord Jestocost

Similar threads

Graduate Golf ball and Tennis ball turbulence
  • · Replies 7 ·
Replies
7
Views
4K
Using golf ball dimples to decrease vaccuum drag on motor vehicles and aircraft.
  • · Replies 31 ·
2
Replies
31
Views
25K
Difficulty Understanding 'Drag Crisis' (transitionary period from laminar to turbulent flow)
  • · Replies 1 ·
Replies
1
Views
1K
What phenomenon occurs that reduces drag when a swim cap is applied
  • · Replies 10 ·
Replies
10
Views
3K
Questions involving airflow and pressure around a car
  • · Replies 21 ·
Replies
21
Views
4K
Need Help Understanding Turblance / Laminar Flow
  • · Replies 11 ·
Replies
11
Views
4K
A question on boundary layers viscosity and air seperating frmo a ball
  • · Replies 12 ·
Replies
12
Views
6K
Prevent Flow Separation | Impact on Drag & Efficiency of Cars & Wings
  • · Replies 8 ·
Replies
8
Views
7K
Aerodynamics: Drag & Bernoulli in Turbulent Flows
  • · Replies 6 ·
Replies
6
Views
4K
Surface roughness in 2D sinusoidal corrugation
  • · Replies 1 ·
Replies
1
Views
5K