- #1
Andrew Jacobson
- 6
- 0
Hi, I'm attempting to do a question involving blood flowing through a blood vessel and I'm incredibly stuck and would appreciate some help. The question is as follows:
'A simplified model of blood flow through the human body makes the approximation that the flow I is proportional to the pressure differential ΔP between any two points of the system ΔP∝I.
Take a length of vessel L with diameter d and viscosity η and let v(r) be the velocity as measured by r from the central axis. Assuming that the blood flow is laminar we can then model:
v(r)=ΔP((d2/4)-r2)/4ηL
The viscous force Fv acts on any cylindrical element due to the slow moving blood outside the element. The magnitude is given by:
Fv=-ηAdv/dr where A=2πrL.'
'(a) Sketch v(r) and then calculate the average velocity through the vessel'
I sketched it as a negative x^2 graph except it doesn't go below the x axis. For the average velocity I integrated to find the area under the curve and then divided by the range. This gave me (ΔPd2)/(24ηL)
'(b) calculate the flow through the vessel'
I said that the flow = ∫v(r)2πrdr between d/2 and 0 and get the answer (ΔPπd4)/128ηL
'(c) Calculate the force on the walls of the vessel'
Here I was a little less sure. I worked out dv/dr to be -(ΔPr)/(2ηL) and then substituted it into the given equation to get F=ΔPπr2 and then subbed in r=d/2 (because it's the force at the wall) to get F=(ΔPπd2/4
'(d) What is the net force on the vessel? Show this is consistent with your answer to (c)'
Here is where I'm completely stuck. I've tried making ΔP=dF/dA and solving differentially but I'm extremely confused. If somebody could help with this part (and correct me if I've gone wrong beforehand) then it'd be greatly appreciated.
Thanks in advance.
'A simplified model of blood flow through the human body makes the approximation that the flow I is proportional to the pressure differential ΔP between any two points of the system ΔP∝I.
Take a length of vessel L with diameter d and viscosity η and let v(r) be the velocity as measured by r from the central axis. Assuming that the blood flow is laminar we can then model:
v(r)=ΔP((d2/4)-r2)/4ηL
The viscous force Fv acts on any cylindrical element due to the slow moving blood outside the element. The magnitude is given by:
Fv=-ηAdv/dr where A=2πrL.'
'(a) Sketch v(r) and then calculate the average velocity through the vessel'
I sketched it as a negative x^2 graph except it doesn't go below the x axis. For the average velocity I integrated to find the area under the curve and then divided by the range. This gave me (ΔPd2)/(24ηL)
'(b) calculate the flow through the vessel'
I said that the flow = ∫v(r)2πrdr between d/2 and 0 and get the answer (ΔPπd4)/128ηL
'(c) Calculate the force on the walls of the vessel'
Here I was a little less sure. I worked out dv/dr to be -(ΔPr)/(2ηL) and then substituted it into the given equation to get F=ΔPπr2 and then subbed in r=d/2 (because it's the force at the wall) to get F=(ΔPπd2/4
'(d) What is the net force on the vessel? Show this is consistent with your answer to (c)'
Here is where I'm completely stuck. I've tried making ΔP=dF/dA and solving differentially but I'm extremely confused. If somebody could help with this part (and correct me if I've gone wrong beforehand) then it'd be greatly appreciated.
Thanks in advance.