Unformatted text preview: 1/13/2010 Problem: A 50mm diameter cork ball (specific gravity = = 0.25) is floating in water. How deep will the ball float? r x The total volume of the sphere is: The submerged volume is given by: 4 V r 3 3 1 VS x 2 3r x 3 Ball weight = weight of displaced water: Final result (in root finding form): V VS (1) x 3 3rx 2 4r 3 0 Note: problem adapted from Fausett p 48 General formulae for a cubic equation ( ax3 + bx2 + cx + d = 0): 1 1/13/2010 For cubics using a formula to find the roots is not a practical proposition. Casio calculator still a possibility (enter 3 for "Degree?" and keep in mind that there may be one real and two complex roots). In Matlab cubics are a piece of cake: >> D = 50; >> r = D/2; >> rho = 0.25; >> p = [ 1 (3 * r) 0 (4 * r ^3 * rho)]; >> x = roots(p) x = 71.9846 16.3176 13.3022 As x must be between 0 and D the answer is 16.3176 mm. fzero is just as convenient and has the advantage of given us just the root we want: >> f = @(x) x.^3 3 * r * x.^2 + 4 * r^3 * rho; >> depth = fzero(f, [0 D]) p y p g Two possible ways of plotting the function: >> figure (1) >> fplot (f, [D (2 * D)]); >> figure (2) >> x = linspace (D, 2 * D, 50); >> y = polyval (p, x); % "p" (polynomial coefficients) from previous slide >> plot (x, y); Function polyval accepts the coefficients of a polynomial and an x value. It produces the corresponding y value. The x value may be a vector (as it is here). In this case the result is also a vector. Note: In this case y = f(x) would have exactly the same effect. 2 1/13/2010 Script mfiles can be made more generally useful by having them read in values. The script mfile below reads in the ball diameter and specific gravity, works out how deep the ball will float, and outputs the result in a nicely formatted message. rho = input ('Enter specific gravity: '); D = input ('Enter diameter of ball in mm: '); r = D / 2; f = @(x) x.^3 3 * r * x.^2 + 4 * r^3 * rho; depth = fzero(f, [0 D]); fprintf ('The ball will float %f mm deep.\n', depth); Function input displays the specified prompt message, waits for the user to enter a value, and then returns this value. Aside: What will happen is the specific gravity entered is not between 0 and 1? fprintf ('The ball will float %f mm deep.\n', depth); Format string Format codes: %d %e,%E %f %g integer format integer format scientific format(upper or lower case `e') fixed point decimal format the more compsct of %e, %f Control Codes: \n \t new line tab Format codes may include a field width and the number of decimal places required: %5d display value as an integer in a field 5 characters wide %10.6f display value with 6 decimal places in a field 10 characters wide See text p 48 3 1/13/2010 Problem: Aircraft are typically equipped with a static port and a Pitot tube. The static port measures the static pressure (P) of the airflow past the aircraft and the Pitot tube measures the stagnation pressure (Po) of this flow. If an aircraft's speed (expressed as a Mach number) is known, the ratio of the two pressures (Po/P) can be computed: for M <= 1: for M >= 1: PR( M ) 1 0.2M 2 PR( M ) 1.2 M 2 3.5 3.5 (7 / 6)M 2 1 / 6 2.5 where M is the aircraft's Mach number Typically the situation is the other way around. The pressure ratio is known (from the static port and Pitot tube measurements) and the Mach number must be determined. This is another root finding problem. We need to find M such that
PR( M ) P0 / P 0 Function PR requires a "function mfile". function [ ratio ] = PR( M ) %PR Given a Mach number, returns the corresponding pressure ratio % Inputs: M = Mach number % Outputs: ratio = pressure ratio (stagnation pressure / static pressure) if M <= 1 ratio = (1 + 0.2 * M^2)^3.5; else ratio = (1.2 * M^2)^3.5 * ((7/6)* M^2 1/6)^2.5; end end Such functions are created in the editor window and each function is stored in a file called function.m, where function is the name of the function (e.g. for this function, the file is PR.m). 4 1/13/2010 Two kinds of mfiles: script files : contain "canned" instructions function files: contain a function (file name = function name) Two kinds of functions (that we care about there are more): "anonymous" functions: defined by a single command file functions: generally more complex, defined within a file File functions are comparable to functions in C++ (and other languages) selfcontained units do NOT share variables with the command window accept inputs and produce outputs The initial comments document the function: >> lookfor mach TargetsComms_Machine class to hold information about a machine PR Given a Mach number, returns the corresponding pressure ratio PRv Given a Mach number, returns the corresponding pressure ratio , p gp sfopen Opens a new machine. sfsave Saves a machine. wmachdep Machine dependent values. >> help PR PR Given a Mach number, returns the corresponding pressure ratio Inputs: M = mach number Outputs: ratio = pressure ratio (stagnation pressure / static pressure) The lookfor command looks for functions having the specified keyword in their initial comment line. The help command outputs all of a function's initial comments. 5 1/13/2010 The Matlab "if" is logically equivalent to a C++ "if". Only the details differ. if M <= 1 % round brackets around expression allowed but not required ratio = (1 + 0.2 * M^2)^3.5; else ratio = (1.2 * M^2)^3.5 * ((7/6)* M^2 1/6)^2.5; end Relational and logical operators: == ~= < > <= >= >= ~ && || is equal to is not equal to ** different from C++ ** is less than is greater than is less than or equal to is greater than or equal to is greater than or equal to NOT ** different from C++ ** AND note: & is array version OR note: | is array version The script file below plots the pressure ratio for Mach numbers from 0 to 7 (world record: the X15 rocket plane achieved Mach 6.72), reads in a pressure ratio, and outputs the corresponding Mach number. figure (1); fplot (@PR, [0, 7]); xlabel ('Mach Number'); ylabel (' l b l ('Pressure Ratio'); ') grid on; ratio = input ('Enter pressure ratio: '); % define function for root finding f = @(M) PR(M) ratio; M = fzero (f, [0 7]); M = fzero (f [0 7]); fprintf ('The mach number is %f\n', M); Note: When a file function is used as an input to another function, the name of the function must be preceded with a `@'. 6 1/13/2010 Function PR is not vector friendly. Unlike most Matlab functions, it will not work properly if it is given a vector of inputs values. In order to convert a vector of inputs into a vector of outputs it is necessary to use a loop: M = 0 : 0.1 : 7; % from 0 to 7 in steps of 0.1 M 0 01 7 %f 0t 7i t f01 for k = 1 : length(M) % from 1 to length(M) in steps of 1 ratio(k) = PR(M(k)); end figure (1) plot (M, ratio); xlabel ('Mach Number'); ( Mach Number ); ylabel ('Pressure Ratio'); grid on; Vectors grow automatically as new elements are assigned values. The function can be made to handle vectors: function [ ratio ] = PRv( M ) %PRv Given a Mach number, returns the corresponding pressure ratio % Vector friendly version of PR % Inputs: M = Mach number (may be a vector of Mach numbers) % Outputs: ratio = pressure ratio (stagnation pressure / static pressure) ratio = ones(size(M)); % preallocate for efficiency for k = 1:length(M) if M(k) <= 1 ratio(k) = (1 + 0.2 * M(k)^2)^3.5; else ratio(k) = (1.2 * M(k)^2)^3.5 * ((7/6)* M(k)^2 1/6)^2.5; end end d end Initializing a vector to its final size (preallocation) improves efficiency. 7 1/13/2010 Ways of creating vectors: >> v1 = linspace (2, 6, 5) % 5 values between 2 and 6 v1 = 2 3 4 5 6 >> v2 = 2 : 6 % steps of size 1 are the default v2 = 2 3 4 5 6 2 3 4 5 6 >> v3 = 2 : 1 : 6 v3 = 2 3 4 5 6 >> v4 = [2 3 4 5 6] % values may be separated with commas v4 = 2 3 4 5 6 >> v5 = ones([1 5]) v5 v5 = 1 1 1 1 1 >> v6 = zeros([1 5]) v6 = 0 0 0 0 0 >> v7 = ones(size(v4)) % size returns a two element vector (# rows, # cols) v7 = 1 1 1 1 1 Matlab also has while loops (just like C++). The script below uses an infinite loop to process pressure ratios until a ratio that is less than 1 is entered. while true % true is 1, false is 0 while true % true is 1 false is 0 ratio = input ('Enter pressure ratio: '); if ratio < 1; break; end % could be return instead of break % define function for root finding f = @(M) PR(M) ratio; M = fzero (f, [0 7]); fprintf ('The mach number is %f\n', M); end 8 ...
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This document was uploaded on 04/14/2010.
- Winter '09