# Search results

1. ### How to Solve dx/dt = Adx/dy + Bdx^2/dy^2

Thanks, that did the trick; nothing like forgetting day one of PDE class!
2. ### How to Solve dx/dt = Adx/dy + Bdx^2/dy^2

Hello, I am trying to solve the following equation: \frac{\partial x}{\partial t} = A \frac{ \partial x}{\partial y} + B \frac{\partial^2 x}{\partial y^2} I know how to solve the diffusion equation (i.e. no dx/dy term), but that method doesn't work here. I tried to go with the LaPlace...
3. ### Finding zeros

Thanks, I hadn't thought to substitute. I got it.
4. ### Finding zeros

I haven't done this in ages, and I'm having trouble recalling how to factor a higher order polynomial. I almost always do this graphically, but for this case I'm interested in an algebraic solution. Specifically, I'm looking at ax + x^3 - x^5 = 0 (with a = an integer >0.) Clearly 0 is one...
5. ### Help with a simple integral

I think I have it - this means that: \frac {1}{v} \frac {1}{v+b} = \frac {A(v+b)+Bv}{v(v+b)} This would imply, since the denominators are the same, that: 1 = A(v+b)+Bv. I can then solve for A and B by choosing easy values for v. So, if v=0, then A=1/b. If v=-b, then B=-1/b. Plugging that...
6. ### Help with a simple integral

In my continued pursuit of isothermal equation of state solutions, I've come upon a very simple integral I can't recall how to do, and was looking for assistance. It is: du = \int \frac{a}{T^{0.5}v(v+b)}dv where a, T and b are constants. so: du = \frac{a}{T^{0.5}} \int \frac{1}{v(v+b)}dv...
7. ### Simple Integral Help

Ah, I see. Thanks for the help! (of course, since you are familiar with the physics, Gib z, you may see I'm trying use the relation of your namesake to fund du, and I could do the attraction potion which is the ultimately simple a/v^2. Again, I greatly appreciate the help!
8. ### Simple Integral Help

I'm trying to integrate the Van der Waals equation of state for an isothermal problem, but based on my results I think I'm doing a bit of simple calculus wrong and hope someone here can help. P = \int \frac{RT}{v-b} dv where R, b and T are known constants. I tried to do a u-substitution...
9. ### Integration Help

Thanks for the help!
10. ### Integration Help

Simple Diff Eq Help I am trying to solve the following Diff Eq: \frac{d^2x}{dy^2}+(\frac{y}{2}-\frac{1}{y})\frac{dx}{dy}=0 I tried to solve by setting \frac{dx}{dy}=z so: \frac{dz}{dy}+(\frac{y}{2}-\frac{1}{y})z=0 I know the general solution to this is...
11. ### Index transformation

Okay, so looking at this backwards makes sense. Since w is general, and thus can be f(x,y,z,t) \frac {D \frac{\omega^2}{2}}{Dt} differentiated by the chain rule gives: \frac{2 \omega}{2} \frac{D \omega}{Dt} = \omega \cdot \frac{D \omega}{Dt} So, looking at it backwards makes it...
12. ### Help with fluid dynamics with differntial equations

What I would try is: 1. Come up with a time dependant expression of the volume of water in the bottle, based on the starting volume of water minus the volumetric flow rate of water leaving the bottle. 2. Then, find an expression for the hydrostatic pressure the water is exerting on the...
13. ### Girl trouble.

Try being nice to her. But talk about your girlfriend anytime you see her. She'll get the hint. For some reason I don't comprehend, many girls often enjoy the chase for attention, and the harder it is to get, the harder they pursue it (much as Twisting Edge says.) But, the same sort hate...
14. ### Index transformation

1. Capital D refers to the substantial derivative, in the notation of Stokes. It boils down to: \frac {D()}{Dt} = \frac {\partial ()}{\partial t} + v_i \partial_i () Also, in the index notation of my book, \frac {\partial ()}{\partial t} = \partial_o 2. w is a vector. v is a...
15. ### Index transformation

In my struggles to understand index notation, I am trying to figure out how my book came up with the following transformation. \frac {D \omega}{Dt} \cdot \omega = \omega_j \partial_j v_i \cdot \omega + \nu \partial_j \partial_j \omega_i \cdot \omega = \frac {D \frac{\omega^2}{2}}{Dt} =...
16. ### Questions about Index Notation

coalquay, You are correct that I intended \epsilon_{ijk}. Sorry for the bad notation. But I greatly appreciate the help, I did not think of that approach. Thanks!
17. ### Fighter aircraft maneovres

Also, on newer aircraft performing extreme manuevers, the impact of thrust vectoring can have huge effects on the manuevuer since the propulsive force is no longer in line with the plane's axis.
18. ### Questions about Index Notation

I am playing around with learning index notation for tensors, and I came across the following where C is a 0th order tensor: E_{ijk} \partial_j \partial_k C = 0 I believe this equates to \nabla \times \nabla C. I don't understand why this comes out to 0. Any ideas? Also, I am trying...
19. ### 2nd Order De Solution

Ah, that's a very nice way of framing the equation, I hadn't thought of that. Thanks!
20. ### 2nd Order De Solution

I am familiar with how to solve a second order, non-homogenous DE with constants, i.e. \frac {\partial^2X(t)}{\partial t^2} + \frac{\partial X(t)}{\partial t} = C by first solving the homogenous eqn, then setting the equation equal to a constant, yielding a sol'n of X(t)= Ae^{0}+...
21. ### Solving a Simple ODE from the Navier-Stokes

So, using the hint I get: uf''(r) + (u/r)f'(r) = 0 Assuming an exponential function, f(r) = e^{nr} f'(r) = ne^{nr} f''(r) = n^2e^{nr} Thus un^2e^{rn} + (u/r)ne^{rn} = 0 u(n^2+1/rn) = 0 un(n+1/r) = 0 n = 0, -1/r So: Vz = Ae^{0} + Be^{-1/r} Is that the correct...
22. ### Solving a Simple ODE from the Navier-Stokes

I've reduced a portion of the Navier Stokes to solve a flow problem, and am left with the following ODE: u (\frac {\partial^2 Vz} {\partial^2 r}) + \frac {r} {u}\frac {\partial Vz} {\partial r} = 0 I tried to solve this equation by assuming a power law solution with Vz = Cr^n Which...